The present invention relates to a method for the preparation of a rare earth-based permanent magnet. More particularly, the invention relates to a method for the preparation of a neodymium/iron/boron-based permanent magnet by a powder metallurgical process involving a step of sintering of a powder compact of a magnet alloy of a specified chemical composition of the rare earth-based magnet alloy.
As is well known, the demand for rare earth-based permanent magnets is rapidly growing in recent years by virtue of their very excellent magnetic properties enabling a compact design of electric and electronic instruments with a permanent magnet built therein despite the relative expensiveness of the rare earth-based magnets as compared with ferrite-based and other conventional permanent magnets. Among the various types of rare earth-based permanent magnets, the samarium-based magnets developed in early days are under continuous replacement with neodymium-based permanent magnets or, in particular, neodymium/iron/boron-based magnets because the magnetic properties of the magnets of this latter type definitely exceed those of the former type in addition to the lower manufacturing costs owing to the relative inexpensiveness of the elements constituting the magnets.
As is also well known, the neodymium-based permanent magnets are prepared, like the rare earth-based magnets of other types, by a powder metallurgical process comprising the steps of pulverization of an alloy ingot of a specified composition of the constituent elements, e.g., neodymium, iron and boron, into a fine magnet alloy powder, compression-molding of the alloy powder, usually, in a magnetic field, into a powder compact and a sintering heat treatment of the powder compact as a green body at an elevated temperature under controlled conditions.
It is generally accepted that the magnetic properties of the thus prepared neodymium-based permanent magnets are greatly affected by the process conditions of the step of sintering heat treatment. For example, the residual magnetization of the magnet can be increased by bringing the density of the sintered magnet body as close as possible to the true density of the respective magnet alloy. Needless to say, the density of a sintered magnet body can be increased by increasing the sintering temperature and by extending the time length for the sintering treatment.
These measures to accomplish an increase in the density of the sintered magnet body can not always be applied with success to the neodymium-based permanent magnet having relatively large temperature dependence of the coercive force because an increase in the sintering temperature and/or extension of the sintering time results in undue growth of the sintered grains while coarser sintered grains have a lower coercive force than finer grains as a trend. This problem explains the residual magnetization of the neodymium-based magnets currently under use which is substantially lower than the value expected ed for an imaginary magnet having a sintering density identical to the true density of the magnet alloy.
In this regard for accomplishing a high residual magnetization, a proposal is made in Japanese Patent Publication 4-45573 for a measure of bringing the density of a sintered body of a neodymium-based magnet to a value close to the true density of the alloy with a relatively small decrement of the coercive force, according to which the density of the sintered magnet can be increased by conducting the compression molding of the magnet alloy powder by using a hot hydrostatic press under a hydrostatic pressure of 500 to 1300 atmospheres. Needless to say, a large problem involved in this method of high-pressure hydrostatic compression molding is that the hydrostatic pressure can be obtained only by using a very highly pressure-resistant vessel which is, even by setting aside the large weight and expensiveness, under strict legal regulations for safety and must be used and maintained with utmost care. In addition, this hydrostatic molding method is disadvantageous due to the low productivity taking a long time for one-shot molding resulting in an increase in the manufacturing costs of the magnet products.
Alternatively, Japanese Patent Kokai 7-335468 proposes a heat treatment under a pressure in the range from 50 to 500 atmospheres to accomplish densification of the sintered magnet body. Although the pressure can be substantially lower than in the above described method proposing a pressure of 500 to 1300 atmospheres, the disadvantage due to the requirement for a highly pressure-resistant vessel still remains unsolved.
The disadvantage caused by a low density of the sintered neodymium-based permanent magnets is not limited to a decrease in the magnetic properties such as the residual magnetization. Namely, a neodymium-based sintered magnet having an insufficient density as sintered is liable to suffer drawbacks such as low mechanical strengths of the magnet body, rusting on the surface and poor adhesive bonding of the rustproofing coating layers provided on the magnet surface.